1. Field of the Invention
This invention relates to a process for preventing the formation of gypsum scale in a flue gas desulfurization process.
2. Description of the Prior Art
Hitherto, many processes have been proposed for flue gas desulfurization, in which alkali absorbents such as sodium, potassium, or calcium compounds are used for treating a gas containing SO.sub.2. One of the processes, which has found a wide application, is a lime-gypsum process using a calcium system, which uses Ca(OH).sub.2 as an absorbent, because the lime is low in cost, abundant and hence advantageous from an economic viewpoint and also because of the freedom from a secondary public nuisance and the possibility of reuse of gypsum.
According to the lime-gypsum process, an exhaust gas is brought into contact with an absorbing solution containing Ca(OH).sub.2 in a scrubber whereby SO.sub.2 contained in the exhaust gas reacts with Ca(OH).sub.2 and is absorbed. According to this reaction, there may be obtained CaSO.sub.3 .multidot.1/2H.sub.2 O, part of which in turn is oxidized so as to give gypsum. EQU Ca(OH).sub.2 +SO.sub.2 .fwdarw.CaSO.sub.3 .multidot.1/2H.sub.2 O EQU CaSO.sub.3 .multidot.1/2H.sub.2 O+3/2O.sub.2 +aq.fwdarw.CaSO.sub.4 .multidot.2H.sub.2 O
When Ca(OH).sub.2 is present in an excessive amount in the absorbing solution, unreacted Ca(OH).sub.2 remains in the absorbing solution discharged from the scrubber, and is transported into the treatment section. This leads to a loss of the alkali.
For preventing the loss of the alkali, it is mandatory to eliminate Ca(OH).sub.2 in the absorbing solution discharged from the scrubber, i.e., to adjust the concentration of Ca(OH).sub.2 contained in the absorbing solution supplied to the scrubber to a level equivalent to or less than that of the SO.sub.2 contained in the exhaust gas, thereby maintaining the pH value of the absorbing solution from the scrubber, at a level of not more than 7. In this respect, the concentration of Ca(OH).sub.2 to be used may be estimated according to the following equation: ##EQU1## wherein G: amount of exhaust gas (Nm.sup.3 /hr)
S: concentration of SO.sub.2 to be absorbed (ppm) PA1 L: amount of absorbing liquid (1/hr) PA1 C: concentration of Ca(OH).sub.2
L/G represents a liquid-to-gas ratio (1/Nm.sup.3), which is generally governed by the performance of the scrubber.
U.S. Pat. No. 3,959,441 filed by the inventors discloses a flue gas desulfurization process using Ca(OH).sub.2, in which CaCl.sub.2 is used for increasing the solubility of Ca(OH).sub.2 and for clearing the absorbing solution. The concentration of Ca(OH).sub.2 in this case ranges from 0.3 to 0.8% by weight.
In contrast thereto, the inventors have not adopted CaCl.sub.2 to increase the solubility of Ca(OH).sub.2 for the concentration of Ca(OH).sub.2 remains relatively low and the loss of the alkali is taken into consideration. To our great surprise, however, when Ca(OH).sub.2 is in a low concentration range, there arises an increase in the oxidation rate of calcium sulfite into gypsum due to the lowered pH of the absorbing solution, with the result of the formation of gypsum scale over the inner surfaces of the scrubber and piping. This causes an increase in pressure drop of the gas within the scrubber and lowers the absorption efficiency of SO.sub.2 due to a decreased amount of absorbing solution. In addition, continuous stable operation of the apparatus has been interrupted due to clogging of the piping. The study of the inventors reveals that gypsum scale is formed when gypsum produced in the scrubber crystallizes in the form of a solid from a liquid, and then clings to the inner surfaces of the scrubber and piping in the form of crystal nuclei, and then gypsum crystallizes subsequently on the crystal nuclei thus formed, so that gypsum crystals grow and form extremely hard scale.
The formation of gypsum scale is inevitable in the desulfurization treatment according to the lime-gypsum process. For this reason, many solutions to the problem have been proposed, in which gypsum particles serving as seed crystals are placed into the absorbing liquid to allow uniform sprinkling of absorbing liquid over the entire surface of the side walls of the scrubber, or in which the amount of the absorbing solution is increased to stabilize the pH value of the absorbing solution in an attempt to prevent the crystallization of gypsum from a super-saturated state. However, these attempts dictate the use of large amounts of absorbing solution, resulting in an increase in the cost of equipment and operating expenses with economic disadvantage.